Download Did the Transgondwanan Supermountain trigger the explosive

+ MODEL
ARTICLE IN PRESS
Earth and Planetary Science Letters xx (2006) xxx – xxx
www.elsevier.com/locate/epsl
Did the Transgondwanan Supermountain trigger the explosive
radiation of animals on Earth?
Richard J. Squire a,⁎, Ian H. Campbell b , Charlotte M. Allen b , Christopher J.L. Wilson a
b
a
School of Earth Sciences, The University of Melbourne, Victoria, 3010, Australia
Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia
Received 22 September 2005; received in revised form 12 May 2006; accepted 17 July 2006
Editor: V. Courtillot
Abstract
The explosive radiation of animals on Earth during the late Early Cambrian period (∼ 530–510 Ma) coincides with the
deposition of enormous volumes of continentally derived sedimentary rocks throughout Gondwana. We show here, that these
quartz-rich sedimentary units, collected from five continents, display remarkably similar detrital-zircon U–Pb age-patterns and
propose that they were sourced from either side of a N 8000-km-long and generally N 1000-km-wide mountain chain (the
Transgondwanan Supermountain), which formed following oblique collision between East and West Gondwana, commencing at
∼ 650 Ma. The depositional system supplied by this mountain chain was N 100 km3, which is equivalent to covering all 50 states
of the USA with ∼ 10 km of sediment, and it lasted for at least 260 Myr. The enormous size of the vegetation-free mountain
chain, its position close to the equator and the dramatic changes in global plate-motion in response to the cessation in continent–
continent collision, together with the possible appearance of biota in the soils that promoted rapid chemical weathering, resulted
in extreme erosion and sedimentation rates that are arguably the highest in the geological record. This led to an unprecedented
flux of P, Fe, Sr, Ca and bicarbonate ions into the oceans. The addition of Sr was responsible for seawater 87Sr/86Sr building up to
the highest levels in Earth's history, whereas the addition of P and Fe provided the essential nutrients that supported a bloom of
primitive life that in turn provided abundant food to support the Cambrian explosion of life. The addition of Ca and bicarbonate
ions increased CaCO3 supersaturation in the oceans, which allowed species in numerous phyla to simultaneously develop
skeletons.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Gondwana; Detrital zircons; Tectonics; Cambrian Explosion
1. Introduction
The sudden appearance and explosive radiation of
animals on Earth during the Late Ediacaran to Early
Cambrian periods (∼575 to 510 Ma) [1–4] represent one
⁎ Corresponding author. Present address: School of Geosciences,
Monash University, Clayton, Victoria, 3800, Australia.
E-mail address: [email protected] (R.J. Squire).
of the most remarkable biological episodes in our planet's
history. These catastrophic biological changes occurred
coincident with dynamic plate-tectonic activity associated with amalgamation of the supercontinent Gondwana
[5–8] and a variety of dramatic changes in global environmental conditions including a rapid rise in Sr isotopic values of seawater to the highest-known levels in
Earth's history [9–13], the escalation in atmospheric
oxygen levels [14–17], over a dozen large fluctuations
0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2006.07.032
EPSL-08308; No of Pages 18
ARTICLE IN PRESS
2
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
in inorganic δ13C values [2,13,18] and major glaciations that included the youngest ‘snowball Earth’ event
[4,19,20]. However, the timing of these biological,
tectonic and environmental events display only broad
overlap (Fig. 1), and have resulted in considerable uncertainty and varied interpretations about the trigger of
the explosion of animal evolution on Earth: the aftermath
to rapid climatic fluctuations between global glaciations
and the extreme greenhouse conditions following
‘snowball Earth’ [19], the Acraman meteorite impact
[21,22]; a substantial (∼ 30°) and rapid decrease in
orbital obliquity [23]; short-lived (∼15 Ma) episodes of
anomalously fast plate motions (i.e., inertial interchange
true polar wander (IITPW) [3]); repeated methanerelease thermal cycling events induced by IITPW [2]; a
dramatic shift in terrestrial weathering processes that was
accelerated by the expansion of an early land-based
biosphere [24]; and changes in accumulation rates of
continentally derived sediment during supercontinent
amalgamation [9].
The two most important biological events related to
the explosion of animal evolution are the appearance of
Ediacaran biota at ∼575 Ma [1,4] and the sudden peak in
diversification of genera in the Botoman stage (∼515 Ma)
[2,3]. The 60-million-year gap between these events suggests that the cause of this radical biological episode was
long-lived and rapidly evolving. While the Acraman meteorite impact occurred broadly coincident with the appearance of the Ediacaran biota [21], it remains unclear
whether this event triggered a drastic change in evolution
when recoveries from other large impacts during Earth's
history have had a relatively minor effect (i.e., addition
of no new phyla). The ‘snowball Earth’ hypothesis, which
predicts a ‘bottleneck and flush’ style of evolution,
also struggles to explain how it influenced this major
biological episode because of the large time break between the youngest-known ‘snowball Earth’ event (Marinoan Glaciation at ∼635 Ma [20]) and the first-known
appearance of animals about 60 Ma later [1,4]). Moreover,
similar microfossil assemblages have been observed both
pre- and syn-glaciation in the USA [25] and pre- and postglaciation in Australia [21], indicating relatively high
levels of survivorship of many species during and after
3
these glaciations. Numerical simulations of changes in
orbital obliquity over the last 800 Ma indicate that they
were all less than 4° [26], thus substantial (∼30°) variations in the Ediacaran–Cambrian period (e.g., [27,28])
are unlikely and probably had little impact on Earth's
biological evolution. Of the remaining hypotheses for the
trigger of the explosive radiation of animals, most involve
an interaction between the dramatic tectonic and environmental events during this interval. However, establishing the temporal links between these events has been
complicated by uncertainty about the timing of amalgamation events in Gondwana and the geometry of the fragments involved [5,6,8,29–31].
One way to trace the history of Gondwanan amalgamation, and the mountains that formed during collisional
orogenesis, is to trace the history of the sedimentary rocks
derived from the uplifted terrane using detrital-zircon
ages. Early Palaeozoic (∼543–390 Ma) continentally
derived quartz-rich turbidites occur extensively throughout Gondwana [7,32–38] (Figs. 2 and 3). The huge volume (∼15 Mkm3) of the continentally derived Cambro–
Ordovician sedimentary rocks in Arabia and northern
Africa, led Burke and Kraus [34] to suggest that the
detritus was derived from mountainous highlands that
formed during the assembly of Gondwana. In marked
contrast, the large volumes of Cambro–Ordovician
quartz-rich turbidites deposited along the proto-Pacific
margin of Gondwana (i.e., Australia, Antarctic, and New
Zealand) are generally considered to represent the product of convergent-margin orogenesis [37–40]. Here, we
present correlations among these widely dispersed quartzrich sediment piles of similar ages from the early
Palaeozoic using the U–Pb age spectra of detrital zircons
from the quartz-rich sedimentary rocks: similar detritalzircon age patterns indicate similar sources, whereas different age patterns imply different sources. Previous
studies on detrital-zircon age data for Early Palaeozoic
quartz-rich turbidites from the proto-Pacific margin of
Gondwana [37,40–42] have identified two main age
populations: ∼650–500 Ma and ∼1200–900 Ma. Williams [43] and Williams et al. [44] pointed out that these
ages match closely the 650–500 Ma and 1200–900 Ma
rocks from eastern Africa, and that the broadly similar age
Fig. 1. Major geological, biological and environmental events related to development of the Transgondwanan Supermountain and Gondwana Superfan System during the late Neoproterozoic to Ordovician interval. Cross-hatched boxes represent the timing and duration of collisional orogenesis
associated with diachronous East–West Gondwana convergence [5,60] (highlighted by the large arrow). The intervals recording the greatest flux of
quartz-rich sediment associated with the Gondwana Super-fan System are stippled in blue, whereas those with lower depositional rates are shown in
grey; dashed lines during the Ediacaran period indicate that the depositional rates are inferred (i.e., sedimentary units not preserved). Remaining data
are from the following sources: major episodes of Neoproterozoic glaciation [4,20,90]; Sr-isotope values for seawater [9,12]; carbon isotope values
for carbonate [2,18], diversity of genera [2]; and episode of rapid plate motion [79]; N–D, Nemakit–Daldynian; To, Tommotian; A, Atdabanian; B,
Botoman; T, Toyonian; Mid, Middle.
ARTICLE IN PRESS
4
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
5
Fig. 3. Time-versus-space chart showing the distribution and stratigraphic relationships of the Cambrian to Early Devonian quartz-rich successions in
East Gondwana (i.e., India [51], Australia [7,45], New Zealand [42], Antarctica [7,37,41,49], South Africa [50] and northern Patagonia [52]. The
stratigraphic position of detrital-zircon samples discussed is shown. Modified after Squire and Wilson [7]. Tas., Tasmania; N.Z., New Zealand; S.,
South; Amer., America; Delamer., Delamerian; Gramps., Grampian; Melb., Melbourne; Transant., Transantarctic; Mount., Mountain.
spectra for the sedimentary units indicated provenance
from the same cratonic region. However, several points
remain unclear about the Gondwanan quartz-rich sedimentary rocks. What triggered the influx of detritus from
Africa? What determined the extent and longevity of the
overall depositional system? Were there environmental
implications associated with the rapid deposition of a
massive volume of quartz-rich detritus? And, what is the
Fig. 2. Simplified location maps. (a) Pre-Jurassic configuration of East and West Gondwana, showing the main tectonic elements and known
distribution of early Palaeozoic quartz-rich sedimentary units [6,7,33,37,51,52]. (b) Simplified geological map of the western Lachlan and eastern
Delamerian orogens in southeastern Australia [7] showing the distribution of the main early Palaeozoic units and the location of detrital-zircon
samples analysed for this study.
ARTICLE IN PRESS
6
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
Fig. 4. Summary of the detrital-zircon components from East Gondwana. Detrital-zircon U/Pb age spectra for quartz-rich sandstones from this and
previous studies (A [32], B [51], C [41], D [53], E [42], F [49] and G [54]). n, number of analyses.
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
significance of this sediment pile with respect to the
broadly coincident biological and tectonic changes?
In this study, we correlated the early Palaeozoic quartzrich sedimentary rocks throughout Gondwana to assess the
tectonic implications of the sediment-dispersion system.
Special emphasis is placed on new data from quartz-rich
turbidites from southeastern Australia, because they
provide exposures of late Early Cambrian to Early
Devonian sedimentary units in Gondwana (Figs. 2b and
3). We use this section to delineate different source terranes
and to assess the areal and temporal shifts in provenance. A
new interpretation for the amalgamation of Gondwana is
derived from this approach.
7
younger grains. An analysis is classified as robust and
used to construct age spectra if two criteria are met:
homogeneity and concordance. If the uncertainty in the
isotope relied on for age exceeded three times that expected based on counting statistics, the analysis was
rejected. Zircons that were not concordant at the 10%
level were also rejected. Of the grains analysed, 554
(57%) were robust, based on the criteria described above.
U–Pb and Pb–Pb ratios and ages for the robust analyses
are provided in Supplementary Table 1 and plots of these
data, generated using AGEDISPLAY [48], are shown in
Fig. 4. Precision and accuracy of the analyses are
represented by results of zircon standard R33, presented
at the base of Supplementary Table 1.
2. Methods
3. Results
Eleven samples were selected for detrital-zircon age
dating from early Palaeozoic quartz-rich sandstone in
western Victoria (Figs. 2 and 3), each being composed of
abundant quartz with lesser detrital muscovite, biotite,
feldspar and low-grade metamorphic rock fragments.
Three samples were obtained from locations with tightly
constrained biostratigraphic ages: ST36 (Eastonian 3–4;
∼452–449 Ma); ST39 (Wenlock; ∼428–423 Ma); and
ST47 (Emsian; ∼400–390 Ma). The Grampians Group
(ST77) and Glen Creek Sandstone (ST48) samples have
inferred Late Ordovician to Late Silurian and Middle
Silurian to Early Devonian ages, respectively [45]. The
other samples are late Early to Middle Cambrian or Late
Cambrian in age based on structural and stratigraphic
relationships [7,45].
Zircon ages were determined using laser ablation inductively coupled plasma–mass spectrometry. Some 971
analyses (one analysis per grain) were performed, and
all samples contained broken and rounded zircon grains
suggesting that abrasion occurred during sedimentary
transportation. Standard heavy liquid and electromagnetic techniques were used to separate the zircons from
whole rocks at The University of Melbourne. Several
unknown populations were mounted with standard zircons (TEMORA 2 and R33), and NIST 610 glass, cast in
epoxy, and polished to expose zircon interiors at the
Research School of Earth Sciences, The Australian National University, using the technique reported in Bryan
et al. [46].
We employed a 208Pb-based common-Pb correction
to all ages reported here [47]. Systemic Hg prevents
direct measurement of common-Pb content (204 Pb) so
it is modelled assuming concordance of the Th–U–
Pb system, and that common Pb is the only reason for
discordance. The reported dates are 207Pb/206Pb ages
for grains older than 800 Ma, and 206Pb/238 U ages for
Early and Middle Ordovician zircons dominate the
Middle Silurian to Early Devonian Glen Creek Sandstone
(ST48), whereas the Late Cambrian Monageeta Shale
(ST46) contains abundant ∼550–500 Ma zircons. In
strong contrast, almost all the other samples display a
dominant composite peak at ∼650–550 Ma, a secondary
composite peak at ∼1200–900 Ma and generally minor
Palaeoproterozoic and Archean peaks. However, subtle
variations in the minor age peaks enable several of these
samples with broadly similar age spectra to be distinguished (see below).
3.1. Correlation of Gondwanan detrital-zircon age
spectra
The Early and Middle Cambrian quartz-rich sedimentary units from the Kanmantoo Group in South Australia
[40], Junction Formation in New Zealand [42], lower parts
of the Goldie Formation in Antarctica [37,41,49] and Table
Mountain Group in South Africa [50] all display a
dominant composite peak at ∼650–550 Ma, a secondary
composite peak at ∼1200–900 Ma and minor Palaeoproterozoic and Archean peaks, thus matching closely the
Early and Middle Cambrian Leviathan and Albion formations (ST23 and ST43) in southeastern Australia
(Fig. 4). The Early Cambrian quartz-rich units of the Tal
Group from northern India [51] and the El Jagüelito
Formation from northeastern Patagonia [52] also display prominent composite peaks at ∼650–550 Ma and
∼1200–900 Ma, although the units from India also have
a composite peak at ∼800–650 Ma whereas a major
∼535 Ma peak is present in the sedimentary units from
northern Patagonia. Composite plots of detrital-zircon age
data from Cambrian and Ordovician quartz-rich sedimentary units in southern Israel display broad peaks at ∼650–
ARTICLE IN PRESS
8
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
Fig. 5. A schematic reconstruction showing the major features associated with evolution of the Transgondwanan Supermountain and Gondwana
Super-fan System. (a–d) Oblique convergence between East and West Gondwana from ∼650 and 515 Ma resulted in the establishment of an
enormous mountain chain (Transgondwanan Supermountain). The Gondwana Super-fan System developed as a result of this collisional event and
tectonic events along the proto-Pacific margin of East Gondwana (see Ref. [7]). Deposition associated with the super-fan system lasted for at least
260 Myr. The present-day areal extent of the Bengal and Nicobar fans [36], are shown in black for scale. Palaeogeography of the continents are
modified after Kirschvink et al. [3].
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
550 Ma, ∼1200–900 Ma and ∼800–650 Ma [33], although individual units such as the Amudei Shelomo
Formation in southern Israel [32] have single composite
peaks at ∼650–550 Ma with very few ∼1200–900 Ma
zircons [32]. Despite these minor differences, the presence
of a dominant composite peak at ∼650–550 Ma and
secondary composite peak ∼1200–900 Ma for detrital
zircons from these Early Cambrian quartz-rich successions
strongly links these sedimentary units in East and West
Gondwana during this time. The paucity of Palaeoproterozoic detrital zircons in these units precludes the
presently-exposed Neoproterozoic to lowermost Cambrian
basement rocks to many of these successions as an
important source (e.g., passive-margin quartz-rich successions of the Mitcham Quartzite from Australia [40], the
Kawakot Group from India [53] and the Cobham Formation from Antarctica [41]; Fig. 4). In contrast, Palaeoproterozoic zircons are generally common in the Late
Cambrian quartz-rich sedimentary units from western
Victoria (ST18, ST21, ST46, ST76), and define a dominant peak in the Sticht Range Formation from Tasmania
[54]. Thus a varied source was present in these parts of East
Gondwana during the Late Cambrian.
The Early to Middle Ordovician quartz-rich sandstone of the Adaminaby Group in eastern Australia [45],
Balloon Melange in New Zealand [42] and upper parts
of the Starshot Formation in Antarctica [49] are distinguished from the Early to Middle Cambrian quartz-rich
sandstone of southeastern Australia (e.g., Leviathan
Formation [7]) by significant populations of ∼ 510–
485 Ma detrital zircons (see Fig. 4). The younger zircons
in these sedimentary units represent the involvement of
late Cambrian to early Early Ordovician zircon-bearing
source rocks [45,55]. Deposition of the Gondwanan
sedimentary units in southeastern Australia and New
Zealand generally ceased by the Late Ordovician or
Early Silurian (Fig. 3). The only exception was in the
Melbourne Zone of southeastern Australia, where
deposition of quartz-rich detritus was nearly continuous
from the earliest Ordovician until the Emsian (late Early
Devonian) [45] periods. The upper Silurian (ST39) and
lower Devonian (ST47) sedimentary units in southeastern Australia display a progression towards fewer 510–
485 Ma and more 2000–1550 Ma zircons, recording the
reduced involvement of the late Cambrian source rocks
and increased involvement of older source material.
Not only the time of cessation of sedimentation but
also the initial influx of quartz-rich sandstones dominated
by ∼650–550 Ma and ∼1200–900 Ma zircons varied
across Gondwana (Fig. 3). Deposition of Early Cambrian
quartz-rich sedimentary units including the Tal Group in
India [51], Kanmantoo Group in South Australia [40] and
9
the Amudei Shelomo Formation in southern Israel have
inferred commencement ages between about 535 and
525 Ma, whereas in western Victoria (ST43 and ST23),
New Zealand [42], Antarctica [37,41,49,56] and possibly
South Africa [50], this influx did not start until ∼515 Ma
[7]. In Tasmania, the influx commenced even later at
∼500 Ma [54].
4. The Transgondwanan Supermountain and the
Gondwana Super-fan System
The appearance of quartz-rich sedimentary units displaying the prominent ∼650–550 Ma and ∼1200–
900 Ma composite peaks on what is now five different
continents (e.g., India, Israel, Africa, Australia, New Zealand, South America and Antarctica) during the Early
Cambrian, is the most striking feature of the Early Palaeozoic detrital-zircon age spectra from Gondwana. The
timing of this enormous sediment influx from similar
source-rocks coincides with the build-up in seawater
87
Sr/86Sr ratios to as high as ∼0.7095 [57] during the Late
Cambrian to Early Ordovician periods. The dramatic rise
in Sr isotopic levels for seawater from the beginning of the
Ediacaran period (∼635 Ma) to the highest-known level
in Earth's history, also broadly coincides with the convergence of East and West Gondwana [5] (Fig. 1).
Although this rise in Sr isotope values for seawater has
previously been attributed to the very high erosion-rates
of Pan African (∼650–500 Ma) orogenic belts such as
those separating the Kalahari and Congo cratons [57–59],
we propose that this dramatic change in ocean chemistry
was largely driven by extremely high amounts of erosion
predominantly from the N 8000-km-long and generally
N 1000-km-wide East African–Antarctic Orogen [6] (Fig.
2a; discussed further below). This very large orogen is
composed of a complex association of crustal blocks,
accreted micro-continental fragments and juvenile island
arc rocks that developed during one of the biggest-known
continent–continent collisions in Earth's history [6,8]
following closure of the Mozambique Ocean between
about 650 and 515 Ma [5,60]. Near the Arabian–Nubian
shield, the rocks from this collision zone are generally
low-to-medium metamorphic grades [8,61], whereas
those from East Africa to the Eastern Neptune Ranges
(Antarctica) are typically much higher grade, reaching
granulite facies in places [8,62]. The striking correlation
in ages between the rocks from this orogen and detritalzircon age spectra for the quartz-rich sedimentary units,
clearly shows that the detritus was derived from the
mountains that developed along this long collisional zone.
Furthermore, the size of the East African–Antarctic Orogen is large enough to supply the huge volume of quartz-
ARTICLE IN PRESS
10
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
rich detritus dispersed over such a remarkably large areal
extent. Here, we refer to the surface expression of this
collisional zone as the Transgondwanan Supermountain
to emphasise its palaeogeographic significance.
The river systems that drained to either side of the
enormous Transgondwanan Supermountain, transported
huge sediment volumes and then deposited them in a
series of giant sedimentary fans (Fig. 5), named here the
Gondwana Super-fan System. The proximal parts of this
system were probably large foreland basins, whereas the
distal parts of the system were major intercontinental
basins (e.g., the Mozambique Ocean [5,60]). Although
the actual volume of sediment deposited by the super-fan
system is difficult to estimate, Fergusson and Coney [36]
suggested that the Ordovician quartz-rich turbidites in
southeastern Australia (i.e., the Lachlan Orogen) were
deposited in a fan that had an undeformed areal extent
comparable to the present-day ∼ 30 Mkm3 Bengal Fan
[63]. In addition, Burke and Kraus [34] calculated that
∼ 15 Mkm3 of Cambro–Ordovician quartz-rich sedimentary rocks were deposited in Arabia and northern
Africa. These fans represent less than half the interpreted
areal extent of the Gondwana Super-fan System (Fig.
5d), thus the total volume of the depositional system
must be at least twice the combined size of these two
systems or N90 Mkm3. Alternatively, the inferred areal
extent of the Gondwana Super-fan System during the
Cambro–Ordovician period is at least 3.5 times larger
than the modern-day Bengal Fan (Fig. 5c), which makes
the volume of the Gondwana Super-fan System
N100 Mkm3. This volume of material is comparable to
eroding an average of only 12.5 km of material from the
8000-km-long and at least 1000-km-wide East African–
Antarctic Orogen. This is enough detritus to cover all 50
states of the USA with a layer of sedimentary rocks about
10 km thick.
The eventual shutdown of the super-fan system is
recorded by a sudden reduction in areal extent of the quartzrich sedimentary rocks during the Late Ordovician (Fig. 3),
which broadly coincides with a rapid fall in seawater
87
Sr/86Sr ratios (Fig. 1). The Late Silurian (ST39) and late
Early Devonian (ST47) quartz-rich turbidites in the
Melbourne Zone, Australia, may represent the final
depositional stage of the Gondwana Super-fan System.
Therefore, deposition and reworking associated with the
Gondwana Super-fan System occurred for at least 260 Myr
(i.e., 650 to 390 Ma).
5. New interpretation of Gondwana amalgamation
Recognition of the Transgondwanan Supermountain,
with its extensive and long-lived depositional system (the
Gondwana Super-fan System), has important implications
for interpreting the tectonic evolution of Gondwana.
Orogenesis associated with the collision between northeastern Africa and the Arabian–Nubian Shield at ∼650–
590 Ma [60] probably records the first stage in the
development of the Transgondwanan Supermountain, but
the most extensive development of the mountain chain did
not commence until ∼580 Ma, when the continuation of
oblique convergence between East and West Gondwana
resulted in East Africa colliding with India and Madagascar
[5]. The timing of this event coincided with widespread
drowning of cratonic margins in northern Africa and
Arabia [9]. Boger and Miller [5] suggested that the duration
of convergence along the East African margin was
relatively short-lived (∼580–550 Ma), and that the main
collisional zone between East and West Gondwana was the
∼530–515 Ma Kuunga Suture [29] that mated Antarctica
and Australia with India, Africa and South America (Fig.
2a). However, the enormous volume of quartz-rich
sedimentary rocks deposited either side of the Kuunga
Suture after ∼530 Ma display detrital-zircon age-spectra
that match closely the ages of rocks from the East African–
Antarctic Orogen. Furthermore, if the Kuunga Suture was a
major suture, then amalgamation of Gondwana involved
three large continent–continent collisional events (i.e., the
northeastern margin of Africa, the East African margin and
the Kuunga Suture) in rapid succession; of these, the
youngest two collisional events were anomalously shortlived (∼15 to 30 Ma each) given their great length
(N 4000 km) and highly irregular (curved) margins. These
features, together with the absence of evidence for
significant arc-related magmatism associated with closure
of the oceanic basin separating India and Antarctica,
suggest that the Kuunga suture was not the major
continent–continent collision-zone between East and
West Gondwana and should herein be referred to as the
Kuunga intracratonic zone. Most importantly, in the current
context, the high-strain zones that define the Kuunga
intracratonic zone are locally less than 100 km wide [64–
66] compared with generally N 1000 km for the East
African–Antarctic Orogen [6]. Thus any contribution of
quartz-rich detritus of the Gondwana Super-fan System
from mountains produced during the formation of the
Kuunga intracratonic zone will be trivial compared to those
produced from the East African–Antarctic Orogen.
Despite the remarkably similar detrital-zircon age
spectra for the Gondwana Super-fan System during the
Early and Middle Cambrian, the timing for commencement of the influx of quartz-rich detritus gets younger
from the northeastern margin of Africa towards the protoPacific Ocean. This change of time of the onset of sedimentation was probably the result of oblique East–West
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
Gondwana collision (Fig. 5). We suggest that the irregular
(non-linear) shapes of the N8000-km-long colliding margins and/or oblique convergence between the two palaeocontinental blocks, combined with episodic reorganisation of global plate-motion, produced several pulses of
uplift and basin development that resulted in the major
fluxes of quartz-rich sediment (Figs. 3 and 5). Although
no N 535 Ma quartz-rich sandstones of the Gondwana
Super-fan System are known, sediment deposition probably commenced at ∼650 Ma from mountains produced
by the collision of northeastern Africa and the Arabian–
Nubian Shield, but these sedimentary successions were
continually recycled during progressive convergence between East and West Gondwana and thus are not preserved (cf., development of the SongPan Ghazê following
diachronous collision of North and South China [67]), as
discussed further in the next section.
The Cambrian and Early Ordovician to Early Devonian quartz-rich sedimentary units of the Gondwana
Super-fan System from the proto-Pacific margin of Gondwana may be distinguished by the presence of younger
detritus (i.e., ∼510–485 Ma zircons), although the upper
Silurian (ST39) and lower Devonian (ST47) sedimentary
units display a progression towards fewer 510–485 Ma
and more 2000–1550 Ma zircons. In addition, comparisons of detrital-mica ages for the Early Cambrian and
Ordovician sandstone units in southeastern Australia suggest that the younger packages had undergone a later
thermal event that reset all the detrital micas near the end
of the Cambrian [68]. The thermal pulse may have been
caused by major late Early Cambrian magmatic events
that occurred in response to the dramatic change(s) in
global plate-motions following the cessation in convergence between East and West Gondwana at ∼515 Ma
[5,7]. For example, along the N4000-km-long protoPacific margin of Australia, New Zealand and Antarctica,
large volumes of forearc oceanic crust and back-arc-basin
basalt were generated at ∼515 Ma [7,69,70]. This event
not only established an extensive new depo-centre in to
which the quartz-rich detritus of the Gondwana Super-fan
System was deposited, but it may have also provided the
heat engine for local (i.e., much of the proto-Pacific
margin of Gondwana) crustal melting and thus a modified
source of quartz-rich sedimentary rocks. The progression
towards fewer 510–485 Ma and more 2000–1550 Ma
zircons recorded by the upper Silurian (ST39) and lower
Devonian (ST47) sedimentary units (Fig. 4) may
represent the eventual exhaustion of source rocks containing the 510–485 Ma zircons and erosion of at least part of
the older (pre-Ordovician) quartz-rich basement. It is
important to note that the markedly different detritalzircon age-spectra for Late Cambrian to earliest Devonian
11
quartz-rich sedimentary rocks in Tasmania and the Melbourne Zone (e.g., samples ST46 and ST48 and the Sticht
Range Formation; Fig. 4) suggest an entirely local source
was present in these areas, thus increased levels of mixing
probably occurred during the final stages of deposition
associated with the Gondwana Super-fan System. The
source of this locally derived detritus in Tasmania and the
Melbourne Zone may have been a micro-continent that
collided with the proto-Pacific margin of Gondwana in the
late Cambrian [7].
6. Potential effect of erosion of the Transgondwanan
Supermountain on environmental change
Erosion rates and sediment production are controlled
by a combination of climate, vegetation-cover, biological
activity in the soil and relief. All are important, but the role
of relief is critical. Rocks may weather rapidly in a warm
tropical climate but, in the absence of relief, the weathering products will blanket the decaying rocks with a thick
layer of soil that will protect them from further weathering
and erosion. Relief allows the mechanical erosion of the
protective soils, which is essential for continuous chemical erosion, and it will be most effective when the soils are
devoid of vegetation.
The role of primitive soil biota in accelerating erosion
may have become important after 700 Ma. During Phanerozoic weathering, soil biota promoted chemical weather
and the formation of pedogenic clay minerals by retaining
cations and water in soils, by increasing fluid residence
times [71], and most importantly, by producing organic
acids that attack silicate minerals [72]. Erosion rates in the
presence of appropriate soil biota can be one to two orders
of magnitude faster than abiotic erosion [72–75]. Kennedy
et al. [24] noted that appropriate biota became available in
terrestrial soils by 700 Ma and possible as early as
1000 Ma. They attribute a systematic change through time
from feldspar- and quartz-dominated mudstone and
siltstone in the Precambrian to smectite- and kaolinitedominated claystone, and an increase in kaolinite and
expandable clays at the expense chlorite and illite in marine
mudstone, from ∼650 Ma to the present, to increased soilbiota activity. They suggested that this is due to a
fundamental change in the dominant weathering mechanism from mechanical weathering in the Precambrian to
more efficient biota-assisted chemical weathering after
∼650 Ma. If their argument is correct, the period between
650 and at least 390 Ma may have been unique in Earth's
history for its extreme levels of continental erosion and
sedimentation. Not only was the Transgondwanan Supermountain the largest, or one of the largest, mountain
ranges in the history of the planet (the mountains pro-
ARTICLE IN PRESS
12
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
duced by the Grenville orogeny are the other contender),
but also they were close to the equator (see Fig. 5) and
presumably in a region of high rainfall. Moreover, the
continents were devoid of protective plants that might
have slowed mechanical weathering, and soil biota had
evolved to the point that they could accelerate chemical
weathering. Erosion consumes greenhouse CO2, which
most likely led to global cooling between 650 and 390 Ma
and to higher oxygen solubility in the oceans. Although
high O in the Cambrian ocean may have contributed to the
Cambrian Explosion it would not have been the trigger for
the start of the radiation. If global cooling and increased O
levels in the oceans were the key, the great explosion in
species would have occurred earlier, during one of the
major global glaciations of the Neoproterozoic.
The period between about 900 and 650 Ma involved
break-up of the supercontinent Rodinia, with relatively
minor continent–continent collisions [76,77]. The main
assembly of Gondwana did not commence until closure of
the Mozambique Ocean began at 650 Ma, resulting in the
initial development of the Transgondwanan Supermountain. Erosion of the supermountain started as soon it began
to form, but most of these early sedimentary units were
deposited in the closing Mozambique Ocean (Fig. 5a–b).
As the ocean closed the sedimentary detritus was caught
in the pincer movement of the closing basin, uplifted into
the evolving mountains and continually recycled back
into the ocean basin until closure was complete. There is
no record of the early quartz-rich sedimentary rocks in the
Gondwana Super-fan System because they were eroded
(recycled) soon after they were deposited. Extensive
recycling is important in the present context because it had
a homogenising influence on the quartz-rich detritus of
the Gondawana Super-fan System and explains why the
two dominant peaks (∼650–550 and ∼1200–900 Ma),
which characterise the sedimentary rocks of the super-fan
system, can be recognised in quartz-rich successions from
as far apart as Africa, India, Antarctica, South America,
Australia and New Zealand. The presence of only the
650–550 Ma peak in parts of the Arabian Shield indicate a
different source in which the ∼1200–900 Ma zircons
were possibly less abundant (see Fig. 5c). Sediment
preservation during this late Neoproterozoic period was
limited to occurrences of locally derived detritus that lack
the double peaks and were deposited on stable platforms
(e.g., southeastern Australia [40] and Antarctica [49]).
However, Sr isotopes provide an unambiguous and continuous record of enhanced erosion during this period.
Fig. 1 shows a rapid increase in seawater 87Sr/86Sr ratios
during the Ediacaran, which is interpreted to be due to the
addition of soluble Sr, derived from continental weathering, to the oceans [58,59]. Note that the main increase
starts at about 635 Ma, shortly after the Transgondwanan
Supermountain began to form.
The build-up of preserved quartz-rich successions of
the Gondwana Super-fan System was delayed until
∼535–525 Ma and it continued to 390 Ma. The start of
sediment preservation in the super-fan system coincides
with the final stages of closure of the Mozambique Ocean.
From this time, the pincer movement of the closing
Mozambique Ocean began to first slow and finally stop,
thus removing the mechanism that so effectively recycled
the super-fan detritus in the closing Mozambique Ocean.
As a consequence, preserved sedimentary units now
began to accumulate in the super-fan systems that developed near both ends of the closed ocean (Fig. 5c). By
515 Ma the collision of East and West Gondwana was
complete [5] and the Transgondwanan Supermountain
was at its maximum extent. We suggest that this was a
period of intense erosion, produced by a combination of
steep relief over a large area, a complete absence of vegetation, active soil biota on the continents and high level of
precipitation associated with the position of the Transgondwanan Supermountain close to the equator (see
Fig. 5c). This interval coincided with a sudden rise in the
seawater Sr isotope curve that peaked ∼515 Ma at the
highest value recorded in the geological record, confirming that this was a period of unusually high erosion
(Fig. 1). As an aside, we suggest that the Grenville-aged
orogenies did not produce a comparable 86Sr/87Sr anomaly because erosion rates during this interval were slower,
possibly because terrestrial soil biota had not evolved to
the point that they could breakdown tektosilicates fast
enough to have a significant influence on the 86Sr/87Sr of
the oceans. High 86Sr/87Sr continued throughout the
Cambrian and into the Ordovician as the Transgondwanan Supermountain continued to shed detritus into the
Gondwana Super-fan System (Fig. 5d).
Variations in carbon isotopes in marine carbonates are
used to monitor variations in buried organic carbon, which
is thought to control the oxygen content of the atmosphere.
Organic C is strongly depleted in 13δC relative to marine
carbonate with a fractionation factor of 28.5± 2.0 [16]. As
a consequence, 13δC in marine carbonates increases as the
amount of buried organic carbon in sedimentary rocks
increases. 13δC marine carbonates increased steadily
through the Neoproterozoic, from +1 at 1000 Ma, to a
maximum of about +11, but more typically N 5, from 800
to 650 Ma [18], followed by a period of steady decline in
13
δ between about 650 Ma and 543 Ma. This simple
overall trend in 13δC was punctuated by several short
excursions to negative 13δC, typically from −5 to −10‰,
which correlate with the Sturtian, Marinoan and Gaskiers
glaciations [18]. Each for these anomalies consists of a
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
rapid decline in 13δ followed by a gradual return to
positive values. The Neoproterozoic period was followed
by an episode of widely oscillating 13δC, especially during
the late Early Cambrian period, where at least ten
oscillations of up to 6‰ are recorded [2] (Fig. 1).
The maximum 13δC values for Neoproterozoic marine
carbonates at ∼670 Ma (the Keele Peak [18,78]) is well
above the modern value of 0 and above all Tertiary values,
which do not exceed +3‰, implying a much higher
fraction of buried organic C between 800 and 650 Ma than
today or at any time during the Tertiary. Derry et al. [16]
suggested that three factors control the amount of buried
C: (i) sedimentation rate; (ii) high organic productivity
stimulated by high nutrient supply in the form of
dissolved P and Fe; and (iii) an anoxic water column to
retard the oxidation of buried organic C. The period 900 to
650 Ma is of low seawater 86Sr/87Sr [13] and no major
continent−continent collision events [76,77], so the high
buried C during this interval could not have been due to a
high rate of sedimentation. Derry et al. [16] concluded that
the high 13δC values must therefore be due to an anoxic
water column between 900 and 650 Ma and that the
reduction in 13δC between 650 and 543 Ma was due to an
overturn of the formally stagnant water column by the
Late Proterozoic glacial events. However, the broad shifts
in 13δC though the Neoproterozoic correlate inversely
with 86Sr/87Sr, implying that high sedimentation rates can
lead to low 13δC. We suggest that 13δC is controlled, not
by sedimentation rates, but by sediment isolation rates. As
noted above, 900 to 650 Ma was a major period of rifting
and break-up on Earth, in which no major continent–
continent collisions have been identified. We suggest that
between 900 and 650 Ma, while the continental fragments
of Rodinia drifted apart, continentally derived detritus
accumulated mainly at the rifted margins along which the
continents separated. Sediment accumulation may have
been slow but the sedimentary units that were deposited
were preserved so that they accumulated large amounts
of isotopically light organic carbon, which drove upperocean C to positive values (see also [2]). From ∼650 Ma
the large continental fragments of East and West
Gondwana began to collide forming the Transgondwanan
Supermountain. This was a period of rapid sedimentation, but it was also a period of rapid continental erosion.
The sedimentary units on the former stable shelves of
the continental fragments, which lay within the collision
zone, were uplifted and eroded, and their organic C
oxidized. This would have released isotopically negative
organic C into the atmosphere and upper-ocean and
lowered the 13δC of marine carbonates. As previously
noted, the principal source of sedimentation between 650
and 540 Ma was recycling of the quartz-rich sedimentary
13
units of the Gondwana Super-fan System within the
closing Mozambique Ocean. Although the marked increase in seawater 87Sr/86Sr during this period shows that
it involved rapid sedimentation, and by inference rapid
organic-carbon burial, this material was uplifted and
eroded as fast as it was deposited so that the recycling
process resulted in no net change in the buried organiccarbon budget. The only contribution to the net buried
organic-carbon budget came from the previously undisturbed continental shelf sedimentary rocks that were
pushed into the orogenic zone. We suggest that the end of
the systematic decline in 13δC at 543 Ma marks the point
at which the closing Mozambique Ocean ceased to
receive a significant contribution of new continental shelf
sedimentary units that had not previously been recycled.
The short 13δC excursions associated with glaciations
are probably due to marine transgression–regression
events [9]. Global glaciation events produce extensive
continental ice sheets, which result in a marine regression.
At the end of the glaciation, the ice sheets melt and sea
levels return to pre-glaciation levels. During regressions,
formally shallow sedimentary units on the continental
shelf are exposed to erosion and oxidation, and isotopically light carbon is rapidly released to the atmosphere.
During transgressions the sedimentary successions in the
inner continental shelf are gradually restored and slowly
accumulate isotopically light organic carbon. As a
consequence, glacial events lead to a rapid decline in
13
δC followed by a gradual return to ‘normal’ values.
These glacially associated negative excursions in 13δC are
superimposed on the steady decline in 13δC between 650
and 543 Ma.
The origin of the second-order oscillations in 13δC
during the Cambrian is problematical. Brasier and
Lindsay [9] suggest that positive excursions are due to
marine transgressions and negative excursions to regressions. However the number of documented regressions–
transgressions in this period is small compared with the
number of excursions, making this explanation unlikely.
Kirschvink and Raub [2] suggest an alternate hypothesis in which the 13δC excursions are attributed to the
periodic release of large volumes of methane gas that had
previously been trapped in the sedimentary rocks. Their
hypothesis proposes that isotopically-light organic carbon
accumulated in sedimentary units on the margins of lowlatitude fragments of Rodinia during the Late Neoproterozoic. These organic-rich successions were then moved to
high latitudes where conditions favoured trapping biogenic methane in layers of gas hydrate. Sedimentation
continued until the geothermal gradient pushed these units
out of the clathrate stability field and built up reservoirs of
high-pressure methane. Finally continental drift returned
ARTICLE IN PRESS
14
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
the sedimentary rocks to low latitudes where they were
gradually warmed and subjected to transient stochastic
releases of global-warming, isotopically light, greenhouse
methane into the atmosphere. Kirschvink and Raub [2]
attribute these releases to small seismic events, sediment
failure, sediment erosion or eddy change, perhaps induced
by an episode of rapid plate-motion during the late Early
Cambrian [79]. Our Supermountain hypothesis provides a
simple explanation for the Kirschvink and Raub [2]
stochastic methane burps. We suggest that they resulted
from the periodic release of pockets of hydrocarbons
trapped in sedimentary rocks as a consequence of rapid
erosion of the Transgondwanan Supermountain. Recent
estimates suggest that only 5 to 25% of the global C budget is stored in gas hydrates compared with 50% in fossil
fuels such as coal, oil and natural gas [80]. We therefore
suggest that the light C released during erosion of the
supermountains comes from the oxidation of fossil fuels;
a modern analogue is 7 trillion cubic feet of natural gas,
currently trapped in the Globe area of the New Guinea
Highlands [81], which will eventually be released by
erosion if they are not exploited for commercial purposes.
The burial of organic C has important implications
for the partial pressure of O2 in the atmosphere.
Oxygenic photosynthesis splits the water molecule
through reactions of the form
CO2 þ H2 O ¼ CH2 O þ O2
If the organic C produced by reactions of this type is
buried in the sedimentary successions, and therefore
effectively removed from the system so that it cannot
back-react with O2, the O2 concentration in the atmosphere will increase [82]. The increase in 13δC between
1000 and 670 Ma, followed by an overall decrease
between 670 and 543 Ma is attributed to increased burial
of organic C between 1000 and 760 Ma, followed by a
decrease to 543 Ma. If buried C is the principle factor
controlling the O2 content of the atmosphere, atmospheric
O2 increased during the Neoproterozoic and reached a
maximum at 670 [18,82,83], after which it declined to
543 Ma. Although the O2 content of the atmosphere may
have declined between 670 and 543 Ma it was obviously
still sufficient to sustain metazoans. 13δC for marine
carbonates near the Ediacaran–Cambrian boundary was
close to zero, which is similar to the modern value [2] and
it oscillated around zero throughout the Cambrian. The
decline in 13δC between 670 and 543 Ma, therefore, does
not require the O2 level of the atmosphere to fall below
that required for metazoans.
There is other evidence for an increase in atmospheric
O2 during the Neoproterozoic. Canfield and Teske [15]
report radiation of non-photosynthetic marine sulphideoxidizing bacteria and a marked increase in sulphurisotope fractionation between 1000 and 640 Ma, which
they argue require atmospheric O2 to be at least 5–18% of
the present level.
7. Implication for the explosion of animal-life on
Earth
As noted earlier, the Ediacaran period records the
most dynamic biological episode in Earth's history: the
sudden appearance of soft-bodied filter-feeding Ediacaran biota from ∼575 Ma [1,4]. Complex life requires
two critical ingredients: oxygen and nutrients. Brasier
and Lindsay [9] suggested that the environmental
conditions that triggered this dramatic biological event
were strongly influenced by the extremely high and
rapidly increasing rates of sediment accumulation and
subsidence associated with amalgamation of the supercontinent Gondwana (e.g., [58]). However, we propose
here that the source of the enormous flux of continentally derived sediment, which triggered this catastrophic
biological event, was not the extensive hinterland
margins of Gondwana but the Transgondwanan Supermountain. As we have already stressed, this extensive
mountain range was formed during a unique window in
Earth's history. Not only were they one of, if not the
most extensive mountain chain in the history of the
Earth, but they formed in the high-rainfall region close
to the equator during a time when the continents where
devoid of protective plants and when soil biota may
have evolved to the point that they could accelerate
chemical weathering. The erosion of earlier mountain
belts, such as those associated with the Grenville-age
orogenesis, may have been hindered by the absence of
suitable soil biota and later mountains were protected
from mechanical erosion by vegetation. We suggest that
rapid erosion of the Transgondwanan Supermountain
resulted in the entry of an unprecedented large flux of
dissolved weathering products, including Fe, P, Sr, Ca
and bicarbonate ions, into the oceans. The flux of these
elements exceeded that recorded during any previous
period of Earth's history. This view is supported by the
generally rapid increase in 87 Sr/ 86 Sr during the
Ediacaran and Early Cambrian interval, which is not
seen during any earlier period, including the ∼ 1200 to
900 Ma Grenville-aged orogenesis [13,84]. We further
suggest that this massive build-up of P and Fe in the
oceans, starting at ∼ 650 Ma, provided the critical
nutrients that led to an unprecedented bloom of
primitive life, especially green algal, which in turn,
provide abundant food from which more complex live
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
forms were able to evolve. That is, the period between
∼ 650 and 543 Ma was unique in Earth's history because
this was the first time the P and Fe contents of the oceans
rose to the level where they could sustain abundant
primitive life. More-advanced life forms exploited this
opportunity, multiplied rapidly and evolved. This
argument is supported by the widespread occurrence
of phosphatic sedimentary units [9], together with early
skeletal fossils that first appeared at ∼ 545 Ma [85].
An important element of this argument is that mass
flux of P and Fe from erosion of the Transgondwanan
Supermountain was sufficient to produce a significant
increase in the concentration of these elements in the
ocean. The role of P is critical because it is essential for
life. The residence time for P in the oceans is short, only
105 years [16]. A build-up of P in the oceans therefore
requires a high flux of P into the oceans so that it is
supplied at a higher rate than it is removed. In this
respect, erosion of the Transgondwanan Supermountain
appears to differ from erosion of the mountains
produced by the Grenville-aged orogenesis. As noted
above, there is no recorded increase in seawater
87
Sr/86Sr associated with the 1200–900 Ma event,
implying that the rate of erosion of mountains associated
with this orogenesis was not high enough for the mass
flux of Sr into the oceans to exceed the rate of removal.
The residence time for Sr in the oceans is 2.5 × 106 years
[86], which is an order of magnitude higher than the
residence time for P. If erosion rates where too low to
influence oceanic Sr, they would have had even less
influence on the concentration of P with its lower
residence time. Therefore, we suggest that erosion of
mountains associated with the 1200–900 Ma orogenesis
did not produce the first radiation of large animals
because the erosion rate was too slow to produce a
significant build-up of P in the oceans at that time.
The explosive radiation of marine fauna during the
Early Cambrian occurred coincident with rapid growth of
the Gondwana Super-fan System as the Transgondwanan
Supermountain rotated and drifted close to the equator
(Fig. 5a–c). The marked increase in seawater 87Sr/86Sr at
that time and the high levels attained (Fig. 1), show that
this was a period of extremely rapid erosion of the
supermountain. As argued earlier, erosion of the Transgondwanan Supermountain may have periodically released large amounts of hydrocarbons, including the
greenhouse gas methane, into the atmosphere, which
would have raised surface temperatures. The Late
Paleocene thermal maximum, during which surface waters
rose by 4 to 8 °C, and which is associated with a dramatic
radiation of mammalian fauna, is interpreted to be due the
release of methane stored as, or trapped under, gas
15
clathrate ([2] and references therein). Kirschvink and Raub
[2] point out that increases in surface temperature correlate
strongly with biodiversity and have argued that releases of
methane gas during the Cambrian could have produced a
similar increase in the temperature of surface water and be
responsible for the Early Cambrian radiation. We agree
that this could be a contributing factor.
Rapid erosion of the Transgondawanan Supermountain during the Early Cambrian may also be responsible
for the sudden appearance of skeletal marine fauna.
High erosion rates must have resulted in a marked
increase in the Ca and bicarbonate flux down the rivers
that drained the enormous mountain chain and to a
concomitant increase in seawater CaCO3 supersaturation. This suggestion is supported by a study by Brennan
et al. [87] of halite fluid inclusions, collected from each
side of the Ediacaran–Cambrian boundary, which is
interpreted to show that seawater Ca increases by a
factor of three between 543 and 515 Ma. Furthermore,
Riding [88] found that cyanophytes changed from
silicified throughout most of the Proterozoic to first
weakly calcified in the upper Proterozoic and then to
strongly calcified in the Early Cambrian. Because the
morphological forms of the calcified and silicified
cyanophytes are similar, Riding [88] argued that this
change is more likely to be due to a change in seawater
chemistry than to evolution of the species. The case
becomes stronger when all phyla are considered. The
sudden appearance of skeletal marine phyla from
∼ 545 Ma [85] could be due to evolution of a number
of species advancing to a point where they could
suddenly precipitate CaCO3 (e.g., [89]), but it is more
likely to be due to increasing CaCO3 supersaturation in
seawater making it possible for all phyla to precipitate
CaCO3 for the first time. A simple evolutionary explanation for the sudden appearance of hard bodies in
numerous phyla requires species from all phyla to
simultaneously evolve to the point were they could
produce skeletons; an unlikely coincidence. On the
other hand, the correlation of the appearance of skeletal
phyla with the most rapid period of erosion in Earth's
history and the occurrence of thin-walled skeletons in
uppermost Ediacaran, immediately prior to the start of
the Cambrian, provides strong support for the increasing
CaCO3 supersaturation hypothesis.
Acknowledgements
This work was funded by Leviathan Resources and
by an Australian Research Council Linkage grant. Gordon
Holm and Asaf Raza from The University of Melbourne
are thanked for their thin-section preparation and assistance
ARTICLE IN PRESS
16
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
with zircon separation. We thank J. Brocks, J. Chappell,
M. Harrowfield, P. Vickers-Rich and I. Williams for their
insightful comments and J. Kirschvink and F. Fluteau for
their very constructive reviews.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
epsl.2006.07.032.
References
[1] S.A. Bowring, P. Myrow, E. Landing, J. Ramenzani, J.
Grotzinger, Geochronological constraints on terminal Neoproterozoic events and the rise of metazoans, Geophys. Res. Abstr.
(EGS, Nice) 5 (2003) 13,219.
[2] J.L. Kirschvink, T.D. Raub, A methane fuse for the Cambrian
explosion: true polar wander, C. R. Geosci. 335 (2003) 71–83.
[3] J.L. Kirschvink, R.L. Ripperdan, D.A. Evans, Evidence for a
large-scale reorganization of Early Cambrian continental masses
by inertial interchange true polar wander, Science 277 (1997)
541–545.
[4] G.M. Narbonne, J.G. Gehling, Life after snowball; the oldest
complex Ediacaran fossils, Geology 31 (2003) 27–30.
[5] S.D. Boger, J.M. Miller, Terminal suturing of Gondwana and the
onset of the Ross–Delamerian Orogeny: the cause and effect
of an Early Cambrian reconfiguration of plate motions, Earth
Planet. Sci. Lett. 219 (2004) 35–48.
[6] J. Jacobs, R.J. Thomas, Himalayan-type indenter-escape tectonics model for the southern part of the late Neoproterozoic–early
Paleozoic East African–Antarctic orogen, Geology 32 (2004)
721–724.
[7] R.J. Squire, C.J.L. Wilson, Interaction between collisional orogenesis and convergent-margin processes: evolution of the Cambrian proto-Pacific margin of East Gondwana, J. Geol. Soc. Lond.
162 (2005) 749–761.
[8] R.J. Stern, Arc-assembly and continental collision in the
Neoproterozoic East African Orogen; implications for the
consolidation of Gondwanaland, Annu. Rev. Earth Planet. Sci.
22 (1994) 319–351.
[9] M.D. Brasier, J.F. Lindsay, Did supercontinental amalgamation
trigger the “Cambrian explosion”? in: A.Y. Zhuravlev, R. Riding
(Eds.), The ecology of Cambrian radiation, Columbia University
Press, New York, 2001, pp. 69–89.
[10] V.A. Melezhik, I.M. Gorokhov, A.B. Kuznetsov, A.E. Fallick,
Chemostratigraphy of Neoproterozoic carbonates: implications
for ‘blind dating’, Terra Nova 13 (2001) 1–11.
[11] G.A. Shields, J. Veizer, Precambrian marine carbonate isotope
database: version 1.1, Geochem. Geophys. Geosyst. 3 (2002) 1031.
[12] J. Veizer, Strontium isotopes in seawater through time, Annu.
Rev. Earth Planet. Sci. 17 (1989) 141–167.
[13] M.R. Walter, J.J. Veevers, C.R. Calver, P. Gorjan, A.C. Hill,
Dating the 840–544 Ma Neoproterozoic interval by isotopes of
strontium, carbon and sulfur in seawater, and some interpretative
models, Precambrian Res. 100 (2000) 371–433.
[14] D.E. Canfield, K.S. Habicht, T. Bo, The Archean sulfur cycle and
the early history of atmospheric oxygen, Science 288 (2000)
658–661.
[15] D.E. Canfield, A. Teske, Late Proterozoic rise in atmospheric
oxygen from phylogenetic and stable isotopic studies, Nature 382
(1996) 127–132.
[16] L.A. Derry, A.J. Kaufman, S.B. Jacobsen, Sedimentary cycling
and environmental change in the Late Proterozoic: evidence from
stable and radiogenic isotopes, Geochim. Cosmochim. Acta 56
(1992) 1317–1329.
[17] M.A. Hurtgen, M.A. Arthur, G.P. Halverson, Neoproterozoic
sulfur isotopes, the evolution of microbial sulfur species, and the
burial efficiency of sulfide as sedimentary pyrite, Geology 33
(2005) 41–44.
[18] G.P. Halverson, P.F. Hoffman, D.P. Schrag, A.C. Maloof, A.H.N.
Rice, Toward a Neoproterozoic composite carbon-isotope record,
Geol. Soc. Amer. Bull. 117 (2005) 1181–1207.
[19] P.F. Hoffman, A.J. Kaufman, G.P. Halverson, D.P. Schrag, A
Neoproterozoic snowball earth, Science 281 (1998) 1342–1346.
[20] K.H. Hoffmann, D.J. Condon, S.A. Bowring, J.L. Crowley, U–
Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia; constraints on Marinoan glaciation, Geology 32 (2004)
817–820.
[21] K. Grey, M.R. Walter, C.R. Calver, Neoproterozoic biotic diversification; snowball Earth or aftermath of the Acraman impact?
Geology 31 (2003) 459–462.
[22] G.E. Williams, M.W. Wallace, The Acraman asteroid impact,
South Australia: magnitude and implications for the late Vendian
environment, J. Geol. Soc. Lond. 160 (2003) 545–554.
[23] G.E. Williams, History of the Earth's obliquity, Earth-Sci. Rev.
34 (1983) 1–49.
[24] M.J. Kennedy, M. Droser, L.M. Mayer, D. Pevear, D. Mrofka,
Late Precambrian oxygenation; inception of the clay mineral
factory, Science 311 (2006).
[25] F.A. Corsetti, A.N. Olcott, C. Bakermans, The biotic response to
Neoproterozoic snowball Earth, Palaeogeogr. Palaeoclimatol.
Palaeoecol. 232 (2006) 114–130.
[26] B. Levrard, J. Laskar, Climate friction and the Earth's obliquity,
Geophys. J. Int. 154 (2003) 970–990.
[27] D.M. Williams, J.F. Kasting, L.A. Frakes, Low-latitude glaciations and rapid changes in the Earth's obliquity explained by
obliquity–oblateness feedback, Nature 396 (1998) 453–455.
[28] G.E. Williams, History of the Earth's obliquity, Earth-Sci. Rev.
34 (1993) 1–49.
[29] J.G. Meert, A synopsis of events related to the assembly of
eastern Gondwana, Tectonophysics 362 (2003) 1–40.
[30] J.J. Veevers, Pan–African is Pan–Gondwanaland: oblique convergence drives rotation during 650–500 Ma assembly, Geology 31
(2003) 501–504.
[31] J.J. Veevers, Gondwanaland from 650–500 Ma assembly through
320 Ma merger in Pangea to 185–110 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating, EarthSci. Rev. 68 (2004) 1–132.
[32] D. Avigad, K. Kolodner, M. McWilliams, H.M. Persing, T.
Weissbrod, Origin of northern Gondwana Cambrian sandstone
revealed by detrital zircon SHRIMP dating, Geology 31 (2003)
227–230.
[33] D. Avigad, A. Sandler, K. Kolodner, R.J. Stern, M. McWilliams,
N. Miller, M. Beyth, Mass-production of Cambro–Ordovician
quartz-rich sandstone as a consequence of chemical weathering
of Pan–African terranes: environmental implications, Earth
Planet. Sci. Lett. 240 (2005) 818–826.
[34] K. Burke, J.U. Kraus, Deposition of immense Cambro–
Ordovician sandstone bodies, now exposed mainly in N. Africa
and Arabia, during the aftermath of the final assembly of
ARTICLE IN PRESS
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
Gondwana, Summit 2000 32, Geological Society of America,
Reno, Nevada, 2000, p. 249.
P. Courjault-Rade, F. Debrenne, A. Gandin, Palaeogeographic
and geodynamic evolution of the Gondwana continental margins
during the Cambrian, Terra Nova 4 (1992) 657–667.
C.L. Fergusson, P.J. Coney, Implications of a Bengal Fan-type
deposit in the Paleozoic Lachlan Fold Belt of southeastern
Australia, Geology 20 (1992) 1047–1049.
J.W. Goodge, P. Myrow, D. Phillips, C.M. Fanning, I.S. Williams,
Siliciclastic record of rapid denudation in response to convergentmargin orogenesis, Ross Orogen, Antarctica, Spec. Pap. — Geol.
Soc. Am. 378 (2004) 105–126.
P.W. Haines, T. Flöttmann, Delamerian Orogeny and potential
foreland sedimentation; a review of age and stratigraphic
constraints, Aust. J. Earth Sci. 45 (1998) 559–570.
P.J. Coney, A. Edwards, R. Hine, F. Morrison, D. Windrim, The
regional tectonics of the Tasman Orogenic System, eastern
Australia, J. Struct. Geol. 12 (1990) 519–543.
T.R. Ireland, T. Flöttmann, C.M. Fanning, G.M. Gibson, W.V. Priess,
Development of the Early Paleozoic Pacific margin of Gondwana
from detrital-zircon ages across the Delamerian Orogen, Geology 26
(1998) 243–246.
J.W. Goodge, P. Myrow, I.S. Williams, S.A. Bowring, Age and
provenance of the Beardmore Group, Antarctica; constraints on
Rodinia supercontinent breakup, J. Geol. 110 (2002) 393–406.
R. Jongens, J.D. Bradshaw, A.P. Fowler, The Balloon Melange,
Northwest Nelson; origin, structure, and emplacement, N. Z.
J. Geol. Geophys. 46 (2003) 437–448.
I.S. Williams, Response of detrital zircon and monazite, and their
U–Pb isotopic systems, to regional metamorphism and host-rock
partial melting, Cooma Complex, southeastern Australia, Aust.
J. Earth Sci. 48 (2001) 557–580.
I.S. Williams, J.W. Goodge, P. Myrow, K. Burke, J. Kraus, Large
scale sediment dispersal associated with the late Neoproterozoic
assembly of Gondwana, Abstr. — Geol. Soc. Austr. 67 (2002) 238.
A.H.M. VandenBerg, C.E. Willman, S. Maher, B.A. Simons,
R.A. Cayley, D.H. Taylor, V.J. Morand, D.H. Moore, A. Radojkovic,
The Tasman Fold Belt System in Victoria, Geological Survey of
Victoria Special Publication, 2000, 462 pp.
S.E. Bryan, C.M. Allen, R.J. Holcombe, C.R. Fielding, U–Pb
zircon geochronology of Late Devonian to Early Carboniferous
extension-related silicic volcanism in the northern New England
Fold Belt, Aust. J. Earth Sci. 51 (2004) 645–664.
W. Compston, I.S. Williams, C.E. Meyer, U–Pb geochronology
of zircons from lunar breccia 73217 using a sensitive high massresolution ion microprobe, JGR, J. Geophys. Res., B 89 (1984)
525–534 (Suppl.).
K.N. Sircombe, AGEDISPLAY: an EXCEL workbook to
evaluate and display univariate geochronological data using
binned frequency histograms and probability density distributions, Comput. Geosci. 30 (2004) 21–31.
J.W. Goodge, I.S. Williams, P. Myrow, Provenance of Neoproterozoic and lower Paleozoic siliciclastic rocks of the central
Ross orogen, Antarctica: detrital record of rift-, passive-, and
active-margin sedimentation, Geol. Soc. Amer. Bull. 116 (2004)
1253–1279.
R. Armstrong, M.J. de Wit, D. Reid, D. York, R. Zartman, Cape
Town's Table Mountain reveals rapid Pan–African uplift of its
basement rocks, J. Afr. Earth Sci. 27 (1998) 10–11.
P.M. Myrow, N.C. Hughes, T.S. Paulsen, I.S. Williams, S.K. Parcha,
K.R. Thompson, S.A. Bowring, S.C. Peng, A.D. Ahluwalia,
Integrated tectonostratigraphic analysis of the Himalaya and
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
[69]
17
implications for its tectonic reconstruction, Earth Planet. Sci. Lett.
212 (2003) 433–441.
R.J. Pankhurst, C.W. Rapela, C.M. Fanning, M. Márquez,
Gondwanide continental collision and the origin of Patagonia,
Earth-Sci. Rev. 76 (2006) 235–257.
P.G. DeCelles, G.E. Gehrels, J. Quade, B. LaReau, M. Spurlin,
Tectonic implications of U–Pb zircon ages of the Himalayan
orogenic belt in Nepal, Science 288 (2000) 497–499.
R.F. Berry, G.A. Jenner, S. Meffre, M.N. Tubrett, A North
American provenance for Neoproterozoic to Cambrian sandstones in Tasmania? Earth Planet. Sci. Lett. 192 (2001) 207–222.
M.G. Laird, J.D. Bradshaw, A. Wodzicki, Stratigraphy of the
upper Precambrian and lower Paleozoic Bowers Supergroup,
Northern Victoria Land, Antarctica, Int. Union Geol. Sci., Ser. B
4 (1982) 535–542.
A.J. Rowell, D.A. Gonzales, L.W. McKenna, K.R. Evans, E. Stump,
W.R. Van Schmus, Lower Paleozoic rocks in the Queen Maud
Mountains, revised ages and significance, Int. Symp. Antarct. Earth
Sci. 7 (1997) 201–207.
C.J. Nicholas, The Sr isotopic evolution of the oceans during
the ‘Cambrain Explosion’, J. Geol. Soc. Lond. 153 (1996)
243–254.
L.A. Derry, M.D. Brasier, R.M. Corfield, A.Y. Rozanov, A.Y.
Zhuraviev, Sr and C isotopes in lower Cambrian carbonates
from the Siberian craton: a plaeoenvironmental record during
the ‘Cambrian explosion’, Earth Planet. Sci. Lett. 128 (1994)
671–681.
A.J. Kaufman, S.B. Jacobsen, A.H. Knoll, The Vendian record of Sr
and C isotopic variations in seawater: implications for tectonics and
paleoclimate, Earth Planet. Sci. Lett. 84 (1993) 27–41.
M.G. Abdelsalam, E.-S.M. Abdel-Rahman, E.-F.M. El-Faki, B. AlHur, F.-R.M. El-Bashier, R.J. Stern, A.K. Thurmond, Neoproterozoic deformation in the northeastern part of the Saharan Metacraton,
northern Sudan, Precambrian Res. 123 (2003) 203–221.
R.M. Shackleton, The final collision zone between East and West
Gondwana: where is it? J. Afr. Earth Sci. 23 (1996) 271–287.
S. Muhongo, J.-L. Lenoir, Pan–African granulite-facies metamorphism in the Mozambique belt of Tanzania: U–Pb zircon
geochronology, J. Geol. Soc. Lond. 151 (1994) 343–347.
J.R. Curray, Possible greenschist metamorphism at the base of a
22-km sedimentary section, Bay of Bengal, Geology 19 (1991)
1097–1100.
S.D. Boger, C.J.L. Wilson, C.M. Fanning, Early Palaeozoic
tectonism within the East Antarctic craton: the final suture
between east and west Gondwana? Geology 29 (2001) 463–466.
C.J. Carson, C.M. Fanning, C.J.L. Wilson, Timing of the Progress
Granite, Larsemann Hills: evidence for Early Palaeozoic orogenesis
within the east Antarctic Shield and implications for Gondwana
assembly, Aust. J. Earth Sci. 43 (1996) 539–553.
I.C.W. Fitzsimons, A review of tectonic events in the East Antarctic
Shield and implications for Gondwana and earlier supercontinents,
J. Afr. Earth Sci. 31 (2000) 3–23.
A. Yin, S. Nie, An indentation model for the North and South
China collision and the development of the Tan-Lu and Honam
fault systems, eastern Asia, Tectonics 12 (1993) 801–813.
P.W. Haines, S.P. Turner, S.P. Kelley, J.-A. Wartho, S.C.
Sherlock, 40Ar–39Ar dating of detrital muscovite in provenance
investigations: a case study from the Adelaide Rift Complex,
South Australia, Earth Planet. Sci. Lett. 227 (2004) 297–311.
A.J. Crawford, R.F. Berry, Tectonic implications of Late
Proterozoic–Early Palaeozoic igneous rock associations in
Western Australia, Tectonophysics 214 (1992) 37–56.
ARTICLE IN PRESS
18
R.J. Squire et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx
[70] C. Münker, A.J. Crawford, Cambrian arc evolution along the
SE Gondwana active margin; a synthesis from Tasmania–New
Zealand–Australia–Antarctica correlations, Tectonics 19 (2000)
415–432.
[71] H. Chamley, Clay Sedimentology, Springer, Berlin, 1989 623 pp.
[72] S.A. Welch, W.J. Ullman, The effect of microbial glucose
metabolism on bytownite feldspar dissolution rates between 5°
and 35 °C, Geochim. Cosmochim. Acta 63 (1999) 3247–3259.
[73] W.W. Barker, J.F. Banfield, Biologically versus inorganically
mediated weathering reactions: relationships between minerals
and extracellular microbial polymers in lithobiontic communities, Chem. Geol. 132 (1996) 55–69.
[74] P.V. Brady, R.I. Dorn, A.J. Brazel, J. Clark, R.B. Moore,
T. Glidewell, Direct measurement of the combined effects of lichen,
rainfall, and temperature on silicate waethering, Geochim.
Cosmochim. Acta 63 (1999) 3293–3300.
[75] S.A. Welch, W.J. Ullman, The effect of organic acids on plagioclase
dissolution rates stoichiometry, Geochim. Cosmochim. Acta 57
(1993) 2725–2736.
[76] I.W.D. Dalziel, Pacific margin of Laurentia and east Antarctica–
Australia as a conjugate rift pair: evidence and implications for an
Eocambrian supercontinent, Geology 21 (1991) 598–601.
[77] P.F. Hoffman, Did the breakout of Laurentia turn Gondwanaland
inside–out? Science 252 (1991) 1409–1412.
[78] A.J. Kaufman, A.H. Knoll, G.M. Narbonne, Isotopes, ice ages, and
terminal Proterozoic earth history, Proc. Natl. Acad. Sci. U. S. A. 94
(1997) 6600–6605.
[79] D.A.D. Evans, True polar wander and supercontinents, Tectonophysics 362 (2003) 303–320.
[80] A.V. Milkov, Global estimates of hydrate-bound gas in marine
sediments: how much is really out there? Earth-Sci. Rev. 66
(2004) 183–197.
[81] Oil Search Limited, Annual Report, 2005, p. 96.
[82] D.J. Des Marais, H. Strauss, R.E. Summons, J.M. Haynes,
Carbon isotope evidence for stepwise oxidation of the Proterozoic environment, Nature 359 (1992) 605–609.
[83] A.H. Knoll, J.M. Hayes, A.J. Kaufman, K. Swett, I.B. Lambert,
Secular variations in carbon isotopes ratios from upper
Proterozoic successions of Svalbard and East Greenland, Nature
321 (1986) 832–838.
[84] G. Shields, J. Veizer, Precambrian marine carbonate isotope database:
version 1.1, Geochem. Geophys. Geosyst. 3 (6) (2002), doi:10.1029/
2001GC000266.
[85] M.W. Martin, D.V. Grazhdankin, S.A. Bowring, D.A.D. Evans,
M.A. Fedonkin, J.L. Kirschvink, Age of Neoproterozic bilaterian
body and trace fossils, White Sea, Russia: implications for
metazoan evolution, Science 288 (2000) 841–845.
[86] D.A. Hodell, G.A. Mead, P.A. Mueller, Variation in the strontium
isotopic composition of seawater (8 Ma to present): implications
for chemical weathering rates and dissolved fluxes to the oceans,
Chem. Geol. 80 (1990) 291–307.
[87] S.T. Brennan, T.K. Lowenstein, J. Horita, Seawater chemistry
and the advent of biocalcification, Geology 32 (2004) 473–476.
[88] R. Riding, Cyanophyte calcification and changes in ocean chemistry, Nature 299 (1982) 814–815.
[89] K.J. Peterson, M.A. McPeek, D.A.D. Evans, Tempo and mode of
early animal evolution: inferences from rocks, Hox, and molecular
clocks, Paleobiology 31 (2005) 36–55.
[90] S. Zhang, G. Jiang, J. Zhang, B. Song, M.J. Kennedy, N.
Christie-Blick, U–Pb sensitive high-resolution ion microprobe
ages from the Doushantuo Formation in south China: constraints on late Neoproterozoic glaciations, Geology 33 (2005)
473–476.